2 donors on 1 gift deed--is this legally ok?

Answer to both the queries is affirmative, i.e, both of them can be donor through the same instrument/ docuument of gift deed and You are required (compulsorily) to be present before the Sub-Registrar at the time of registeration of  Gift Deed in your favour.

1. Yes, both can execute and register  one single gift deed as  donors of their share of the property,

2. Gift deed is compulsorily registrable for which you shall have to be presnt before the registrar at the time of registration of the said gift deed.

Yes. Both of them can gift their shares in the property by executing one Gift Deed.

Yes both can be donors to one donee, it is legal and valid, however, instead of executing a registered gift deed, both can relinquish their title by executing a single registered single deed in your favor which will be more effective for the reason that the property is intestate  of your deceased father on which all of the three have equal rights, this will cost you a lesser stamp duty as well enquire about the stamp duty of your state and proceed as per whichever will be cheaper.

In any case you shall have to be present before the registrar to acknowledge the receipt of the said deed in your favor.

Yes, Relinquishment Deed is the best option and economical.