I would add that you may see the initial pit temps come up slower than with a single roast. This is due to the larger mass of cold meat. But once the meat starts to rise, it will probably be like your single roast smokes. It will not really change the cooking process and the times should be pretty close. Just make sure you cook by internal temp and not so much as by time alone.
As others said, if your racks differ in temp, you may want to rotate the meat between them, but it's not like you need to do this every x number of minutes. Also keep in mind that roast A and roast B may just differ in the amount of time each takes to cook even under identical conditions. There are due to the inevitable variations in the weight, meat density, fat, etc... that influence this. One roast may stall and the other may not, etc... Just keep an eye on the temp of both.
About half way through (half way of your normal time for a single roast), I would check the temp of both. If they are close, leave it be. If one is a good bit higher than the other, switch them out on the racks. Then I would check around 85% of the way on what it normally takes. You should have a good idea of how close you are and if they will both be done at the same time or not. If you smoke with a probe constantly in (like with a Maverick or other remote probe), then just keep an eye on the progress.
Also I would start the smoke earlier than normal. This gives you "fudge" time to allow for any variations in the smoke. Nothing like a stall to screw up dinner time!!! Even if both roasts are done a few hours ahead of serving time, you can pull them, tightly wrap in foil and keep them in a cooler for hours (just like we do with pork butts for pulled pork). I've held pork butts this way for up to 5 hours with no problems and they were still too hot to pull by unprotected hands. If you use the cooler, try and select a cooler that will be just about the size of both roasts if you have more than one. If your cooler is too big, fill the voids with some towels to serve as extra insulation around the foiled meat.
So relax, you are going to make some great pulled smoked beef for the gang!!!!! Just plan ahead and think it through.