There are pros and cons with the set up. The thing with double headed chainsaw mills is that you need another power head, more fuel and usually a helper unless you set up both throttles with a single control.
I have heard of others that have run different sized power heads. They will even out when running.
You will get more power and torque, and if using larger saws, there should be no bog down. You will get higher rpm in the cut than a single head so that will give a higher chain speed than a lower rpm/ chain speed.
You wont need an aux oiler .
And you can get faster chain speed . You have the power so larger sized sprockets can be used increasing chain speed and still have reasonable torque because of the double headed set up.
There is a guy on e bay selling 9, 10 11 and 12 tooth sprockets for 090's and 880's .
You will need more chain links in your chain to suit these sprockets.
The one thing with larger sprockets is to not go too big a tooth count and make sure your bar is in good condition.
The larger sprockets will have the chain sitting in the bar groove higher up from the bar heel so with sloppy bar grooves etc there is more chance of chain derail.
If say you had 2 x 880's , running 2 x 10 tooth sprockets that would make a fair difference in chain speed over the standard 7 tooth sprockets.
The bigger the power head, the bigger the sprocket can be used
If you had seriously wide slabs then that would slow down the saw up but not as much as a single head.
So you will have the cost of the bar, another power head, repairs , fuel and probably a helper but hopefully less time milling and breathing in fumes and dust. Also depends on how much time you have / want to spend milling.
Cheers Wayne