Hi Dmgrant1
Sorry if this is an oddball suggestion but if you have the space how about an Accuphase C200 vintage preamp as a phono stage? I have it and it has 2 phono stage inputs. With both you can adjust the volume of each channel of each turntable via the disc volume knobs in the rear. You can also make adjustments to the impedance on one of the turntables via controls in the front. If you wanted to have a fixed output to your existing preamp just use one of the Tape Record outputs. If not just use the existing preamp outputs.
I've seen Accuphase C-200 preamps range in price from $300 to $700 in pristine condition. Check it out. It is a vintage unit but a solid one. Best of luck with the search.