I would recommend having the tank drilled for two 2" bulkhead fitting instead of 1.5". It's very easy to reduce to 1.5", but in case you want more flow through the tank, increasing to 2" would be a pain. Using 2" drains gives you a lot more flexibility.
In regards to the Lagunas, are you buying two so that you have one for a back-up in case it fails? Running both of them will definitely overpower two 1.5" drains...even if you end up going with 2" drains, you may need to valve the 2900's back a little so that they don't overwhelm your drains.
With my setup, the overflow teeth on my overflow towers get overwhelmed before my 2" drains do. In other words, not enough water can get through my overflow teeth quickly enough, so water will spill over from the openings on top if I try to push too many GPH. If I wanted more flow, I could either have the overflow teeth cut down a little lower, or widen the gap between the teeth. This is just something else to keep in mind. I haven't really looked around for this because I can push about 4,500GPH through my system before overwhelming my overflow teeth, but I'm sure there is a flow chart somewhere for how many GPH you can push through X inches of whatever size overflow teeth you will have.