Linda
My 2008 MS 34HT has Sidepower SP95 bow & stern thrusters.
I am very happy with the performance of the pair. I use both for backing into our slip and tend to alternate B & S instead of using either one or the other. They really provide a similar rotation to the boat so either one will provide the rotation. Where having both comes in handy is when doing "fine tuning" you can choose to use either one depending on which end needs tyhe primary correction.
I am happy with both being 12V as mine are both wired to the same thruster 8D bank roughly midship. It is the Mainship OEM location but I have switched to all AGMs aboard. I have recently switch my single eng start from the previous combo house / Start bank of 2 8Ds to my thruster bank 8D. That leaves me with a pure house bank.
I also added a Balmar batty monitor to monitor my house V & SOC as well as thruster bank V.
I would think using one 12V and one 24V would add unnecessary complexity to the batty & charging system. If you have a dedicated bow thruster batty located under the bed then duplicating it in the stern would allow an easier set up but you still need to then have a separate 24V charger.
If your bow thruster batty is MS OEM Im guessing it is located port side near aft end of engine (that's where mine is) and simply adding cable, switch & fusing is all that's req'd to power a second thruster for the stern.
I'll attach a few pics of my thruster wiring that includes alt charging capability.
Details of the Charging System Mods can be found on my Bacchus website - Projects Pg 4
https://dkloeber.wixsite.com/bacchus/project-pg-4
FYI - My stern thruster is mounted off center on port side and includes the tunnel "lids" that helps direct the flow.
Hope that helps - any questions ask away.