Peter, the short answer is, - Don’t do it.
It is very difficult and risky.
This is not a menu item to be served to young children, the elderly, or anyone with a compromised immune system.
Anova states its capacity is “8 meals”, whatever that means. The company no longer recommends the former 5 US gallons, about 40 l, cooking capacity.
These are not commercial cookers and 36 people is definitely in that volume class.
Don’t you have an oven?
If you must proceed, first there are some practical and technical hurdles to clear using two APCs. They draw too much electricity to be on a single domestic circuit. The PID control circuitry could get confused by a second heat source operating at the same temperature. There have been reports of successfully using two units set slightly apart in temperature. You might be better advised to used a large well insulated vessel.
Yes, have the seller vacuum package the skinless filets. Sear or bake the salmon skin to crisp it separately. You will be safer using previously frozen salmon that kills any parasites.
Have you previously used the SV cooking technique?
You don’t reveal the time or temperature you plan to use and that gives me additional safety concerns for your guests.
It is absolutely critical that there be ample water circulation over all food surfaces for safe and effective cooking. Wood floats, use wire racks or bent wire coat hangers to separate fish in a vertical orientation, like they are swimming. Reverse the orientation of adjoining fillets.
Unbrined SV cooked salmon appears poached when not seared. To serve, top portions brushed with a little lemon-herb butter and pieces of crisp skin.
If your pan doesn’t fit the fish, cut the fish to fit the pan.
You don’t tell what type of burner you consider. Avoid propane for its potentially oily taste.
You certainly are persistent like most Dutch people i know.
Yes, there are no rules against using other types of fish as long as you are aware that they are best cooked at different temperatures.