Can an external match-funder, who is not a project partner, agree to contribute match-funding to two organisations that are partners of the same project, but contribute more funds to one than the other?

Yes, this is possible. However, it is not possible that two or more organisations who are partners in the same project match-fund each other (unless they are naturally and structurally linked by existing financial arrangements in the absence of the project). For further explanation and examples, please see section 2.6 of the Programme Manual.