ssh-agent-multiplexer
This is a small program which multiplexes running ssh agents.
If you wanted to aggregate two agents, just do this:
$ ssh-agents-multiplexer --listen multiplex.sock --target agent1.sock --target agent2.sock
2022-09-28T23:06:59+09:00 INF revision=0dbfb04 version=0.0.1-dev
2022-09-28T23:06:59+09:00 INF Agent multiplexer listening listen=/var/folders/2s/41bhrr7d0r76pkkb9kgjtk0h0000gn/T/ssh-agent-multiplexer-90668.sock
Then, an multiplexed agent is listening at multiplex.sock. You can use the socket as an usual ssh agent.
$ export SSH_AUTH_SOCK=/var/folders/2s/41bhrr7d0r76pkkb9kgjtk0h0000gn/T/ssh-agent-multiplexer-90668.sock
$ ssh -A some.host