Might be worth sniffing out local Hackerspace / Makerspace?
They get stuff donated, also members can have hoards of retro gear for "projects" (unfulfilled dreams).
You might have an issue with RAM on the A2091 - you can't set an A2058 to have 6MB... and you can only have 8MB total fast RAM? On a 68000 Amiga?
And you do want some RAM on the A2091 to let it use DMA.
You can have much more than 8 meg on an Amiga, even a older model. I have one machine with 128 meg of ram. One machine with 24 meg of ram, both A2000 systems. One with an accelerator, the other stock 68000. And the A2091's are already populated with 2 meg. I don't need ram for those, only for the A2058's of which I need an additional 6 meg for each card. They already come with 2 meg stock built in and are a maximum of 8 meg once filled.
So I need 6 meg of chips, x 2, for both A2058's I have to fill now, which will bring each of them to their max of 8 meg of ram.
The system as a whole can have plenty more than 8MB, but the Zorro II bus will max out at 8MB. 8MB on the 2058 plus 2MB on the 2091 will cause some issues.
It should not cause any issues if the boards are mapped correctly for memory. As long as they each have their own unique address, you should and can use multiple boards and more ram as fast ram. Not only was this something we did back in the day, but I have a system doing it right now as I mentioned. No issues what so ever. I can populate all that ram with a problem. I AM using only official stock boards in this machine, all z2 and no 3rd party stuff. There IS a caveat to this however.. You need a 4.6 (if I am remembering right from that far back) or better rev motherboard at the least. It might be a Rev 5 series, I will have to dig up some of my tech data from back then to be sure. Rev 6 boards should work fine and the B2000 boards, which I have one of those as well, are going to be a problem because they were never sold. The B2000 boards we gave to dealers ONLY as SHOW units because they have an inherent flaw in the design that has to do with DMA and zorro issues. So they are going to be a problem with a few things for anyone trying to use those for anything more than a standard basic Amiga system.
Another way to get around that, is z3 cards or CPU slot cards or if I remember right, there are even a couple video card slot ram cards that also work.. not sure on this latter one though but I thought I remembered seeing it available somewhere. My other machine has 128 meg ram but I am using the CPU slot for that. The stock machine, I have the 24 meg in because I am using 4096 block CF cards for the drives which eat ram and validation is an issue if you don't have enough of it, as you know.
BTW, did you note I found the other problem while troubleshooting. Bad 7.0 rom burns for the two sets I bought.. Replacements are on the way so for now using my official set in one and the 8.0 set in the main machine I use.