I did some research recently on this:
The following is from Majoor Hans Boersma's site on the 27th Jagers under Tactics: http://www.wargame.ch/wc/nwc/newsletter/November2001/Newsletter16/DutchHistory.html
"As mentioned, Netherlands infantry regulations were merely a translation of the French infantry regulations of 1791. Regarding line tactics however, just prior to the campaign a change of direction was made by the Commandant en Chef [commander-in-chief] of the Army, the Prince of Orange:  on 24 April 1815 he issued the order that from then on the troops, when operating in line formation, should be arrayed in 2 instead of in 3 ranks. Regarding the exact implementation of this order no specific documents are known to exist; in all probability the battalion commanders merely removed the middle rank and distributed its men over the 1st and 3rd (now 2nd) rank. The result was a British- style line formation commanded by a French-style cadre. The change was received with much criticism, and indeed the question arises whether it was sensible to apply such a drastic change to such a young Army with a major campaign looming on the horizon. An official motivation for the adoption of the 2-deep line is not known to me; a variety of reasons has been given by a variety of authors in the past, all of which make some sense: reducing vulnerability to artillery fire; increasing fire power; a necessary increase of unit frontage because of the fact that a lot of battalions were not on full strength; and, in general, British influence through the Prince of Orange, who was a General in the British as well as in the Netherlands Army. Whichever it may be, several after-action reports and eyewitness accounts confirm that Netherlands infantry indeed applied the two-deep line formation during the Waterloo campaign ."
[Note 2] For instance: De Bas/De Wommersom, Vol. III, p. 338; De Jongh, p. 19; Scheltens, p. 84 (also in Coppens/Courcelle: Le Chemin de Ohain, p. 32 and 35 respectively). De Jongh and Scheltens even mention their battalions lying down on the ground at Waterloo, in British fashion.
I had some notes copied from Art Pendragon back in 2000. He mentions the reports from Lt. Shelton (Grenadier company of the 7th Line), Colonel de Jongh (commanding the 8th Militia) and Colonel van Zuylen (Perponcher's chief of staff) as all mentioning forming in 2 ranks. Lt. Shelton mentioned that prior to the campaign they were learning to use a new drill (they were all in Bijlandt's brigade).
In his Armies at Waterloo Scott Bowden references a source of the Netherlands forces being in 2 ranks as Constant Rebecque's (Orange's chief of staff) report of the campaign.
Geert van Uythoven, who has done research on the Netherlands' forces in the campaign, says in an article on the Nassauers that they were being trained to use two-deep lines.
Now for first-hand accounts. One was in one of the Art of War's that Clash of Arms put out. It was van Zuylen's (Perponcher's chief of staff) report, published on Oct. 25, 1815. Bijlandt's brigade on the 18th was positioned between Kempt's and Pack's brigades, with the 7th Line and 7th and 8th Militia in line with the 5th Militia in reserve and the 27th Jagers out front with the 95th Rifles.
"The brigade was in two ranks, due to which the fire was weak and not well maintained, while enemy fire soon opened up gaps through which the enemy could advance."
http://home.scarlet.be/~tsh40803/8/8st.html is the website of the 8th Militia. If you go to the link "Background" on the left you can then download van Zuylen's journal (Dagboek 2de divisie). For the entry of April 27th he says:
"In plaats van op drie gelederen zullen de troepen van nu voortaan op twee gelederen gerangeerd komen. De troepen zoo veel terrein en mogelijkheden toelaten te exerceren in linien en in de linie evoluties te oefenen."
Here's the babelfish translation:
"Instead of on three ranks the troops of now as of now on two will come ranks shunted. The troops zoo much area and possibilities allow practise evolution at exerceren in linien and in the linie."
If you click on the link "8th C.O." on the left, you can download Colonel de Jongh's report. He says:
"Den 18den positie hebbende in den hollen weg achter La Haye Sainte, gedekt door een walletje met enig kreupelhout, halve borstwering, heb ik het hout hiervan doen afhakken, en mijn bataillon, daar achter in twee gelederen geplaatst altoos doen nederliggen om de soldaten voor het vijandelijke kanonvuur te dekken, en bij het aannaderen van de vijandelijke kolonne hen weer doen opstaan, als zij met effect den vijand met hunne geweren konden bereiken."
Here's the babelfish translation:
"Pine 18den has position in pine runs away behind drawer Haye Sainte, covered by walletje with some kreupelhout, half railing, has I it wood of this does chop off, and my bataillon, in two ranks placed altoos do nederliggen cover the soldiers for the hostile cannonade, and at the aannaderen of the hostile kolonne them do stand up, if they with impact pine guns were possible enemy with hunne bereiken."
Not too clear but he mentions that the battalion was in two ranks and they were laying down during the cannonade.
So we know that Bijlandt's brigade was in two ranks at Waterloo. Perponcher's whole division (i.e. Saxe-Weimar's brigade also) was probably in 2 ranks. I couldn't find anything about their ranks at Quatre Bras.