Peter,
To answer the question if it will take 2 keyers the answer is YES, no matter how you build it.
You can accomplish your desired effect several different ways, and all are correct, it depends on your style of using the switcher. Here is how I would do it since I switch my shows mainly using MLE2 (pgm/pvw), while using MLE as my working M/E for thing like 2, 3, 4 boxes, split screens, and other various things.
On MLE1:
If the 2 boxes are stationary. I would use Key 1 as my left box and Key 2 as my right box, assigning each as a dve, and place them where I want them on the screen, I usually use bars as a source when building so I can do center cuts, I then put Key 3 as CG1 and Key4 as CG2, in case I need to add either one over the 2 boxes. On the pgm bus of MLE1 select the source that will be the background, be it a camera, tape machine, or a media player. Then save that as an emem, Example: bank 1 - emem 1. You could also save the MLE using the save MLE snapshot function but I like the emems way.
To recall this you can eiher select the emem directly on MLE1 or you could setup a custom control to recall that emem you just built. If you saved the effect via the snapshot method you could make you custom control recall that snapshot also. Again, any way you choose how to do it is the correct way.
Another way to build this is with the mini-M/Es but it will tie your hands more if you need to be able to select different sorces in your boxes at a moments notice. However, if your sources will NEVER change, you could, for example, make miniME1 background source be your JPG and on Key2 of miniME1 use the dve function, then nest that into miniME2 as the backgroud source an add miniME2 key2 as the second dve
OR you could take miniME1 and make that the background on MLE1 while adding in the second box using any of the MLE keyers. Again it is up to you, and how you wish to build the effect.
If you need further help just post here or email me and I will be glad to help you more.
#Carbonite