Looks like they're stranding the fingering weight together with the bulky stuff for part of the pattern anyway, so if you're planning on doing that part of the pattern, you'll actually be working three strands for part of it if you double up on the main yarn. But you should be able to find a bulky kind of SOMETHING that will give you the right gauge without having to hold 2 strands.
Depending on what kind of work your husband does, you might want to avoid mohair anyway, unless the Lamb's Pride is machine washable. Cascade 128 Superwash would probably do the job, and it's close to the same yardage/weight. Plymouth makes a chunky version of Encore too, but I'm not sure you could get the gauge they specify with that. In my memory, it's a little thinner. But you could probably get away with either one anyway. The designer says it stretches for days, so if it's a little smaller than the given dimensions it probably doesn't matter.