A signature may even need more than two transactions, e. g. if it is a multisig signature.
Yes, the reason is that Winternitz signatures are large (for every tryte you are signing and each key that is part of the multisig you need a full hash length, i. e. 81 trytes in case of KERL, of signature data).
All spending transactions of a bundle sign the same information, the bundle hash.
The bundle hash is computed of the following fields of all transactions of the bundle:
- (Obsolete) Tag
- Time Stamp
- Current Index
- Last Index