just because the filter is rated for 90g, dosent mean it would sufficiently filter a 90g tank with anything but an insanely low sticking level in my opinion. rather than kill yourself with water changes ever 2 days and paying for water conditioner, get a better filter. in which case you can keep a normal schedule of water changes once every week. saves time and money in the long run.
that being said.....for what your going to pay for 2 emp 400's, you could get a decent canister filter for the tank. something extremely easy to maintain like 2-Rena xp3 , or a little more of a hastle to maintain, but is an amazing filter, the fx5. the fx5 can be found for under 200, and if you shop around and look online, you can find the xp3 for around $100 each.
if you insist on nothing but a hang-on-back, i would go with atleast 3 aquaclear 110's, 4 if possible. it may seem like overkill, but cichlids can put a strain on any filter with the bio-load medium-large fish produce.