Hopefully someone with more knowledge chimes in here but from what I have read I would guess that the low frequency response will be similar enough to place the sub in various locations and expect to get useful information about what your room does with low frequency sound.
Meaning that probably the same room modes will be excited now as when the room is finished.
Are you putting a door to the stairwell?
I am not sure how much you like your bass but most likely your dual 8'' in walls times two will not be sufficient for your room. Those subs are probably more suited for use as balancing subs as Blldo sort of alluded to. You might want to consider placing a very powerful sub behind your false wall (make sure you give yourself at least 2 to 2.5 ft). Then experiment with the placement of your in wall subs as balancing subs to smooth the frequency response and help to keep the response even through multiple rows of seating. You would need a way to EQ the subs differently with this method. So if your AVR does not have a way to do this you would need to buy a separate dsp unit.
If you are going to stick with just those two subs then there are many theories as to what gives the best response. Best could mean most powerful (how to take full advantage of room gain to make it go really loud) or best meaning most even frequency response. The oft cited resource is the paper by Welti and Devantier. Here is a link.
There is a cliff notes version with slides somewhere that gives a rundown of best placement scenarios. Basically 2 subs is almost as good as 4. Front and back is a good option. So is midpoints. This strategy is assuming that they will be EQ'd the same.
If you have the time or expertise to fiddle with it a potentially superior solution is to do what I described before. I think one of the people to implement this strategy was Earl Geddes. You can google it.
Here is a link to BG radia in wall subs installation manual. There are some tips on placement. Check out page 3-6 especially.
Now if you are only EQing for one seat it is my understanding that this is far simpler and does not require all of this black art stuff.
Anyway there are tons of resources here. I am not really one of them.