AlgoSigner’s upcoming v2 signing will support atomic transactions which are what you are looking for here. In v1, this is partially supported, but I would urge you to hold off for the two weeks or so before we release v2.
Atomic transactions are described here - Atomic Transfers - Algorand Developer Portal .
How it will work is both transactions will be sent to AlgoSigner, one for the donor to sign and the other an unsigned copy of the NGO transaction used to verify the group. The user signs theirs and it is returned to the dApp where it will be concatenated with the NGO signed transaction and sent in.
The team is available on Discord during the work week and happy to assist. https://discord.gg/YgPTCVk