ND filters and/or zoom all the way in. but there's a reason DSLRs sell because it's hard to fake this look. Can't you just get your hands on another DSLR and/or not to to make the camera be what it's not?
For an upcoming symphonic concert we have 7 DSLRs and a friend of ours has two HPX370s. The issue with the DSLRs is that they can't zoom live which is very crucial for this production. We wanted to use the 370s to accomplish this.
If it were me- I wouldn't worry then about getting them to fake shallow depth of field. Look at any concert film and you'll see a huge variety of depths of field from shot to shot as it progresses. Plan your shooting diagram accordingly and you'll do fine.
Right exactly- I'd make sure to spend some time getting exposure, gamma and color balance similar looking between these cameras. Then you can really nail it in post when you grade. The reality of just about any production these days is multiple camera types, formats, resolutions, etc. Harmonizing all of that in your final edit is nothing to be afraid of, as long as you plan it that way from the beginning of the shoot.