2Phase Commit Protocol

Hi,
Can somebody guide me about the following question related to 2-Phase commit protocol?

If we use two-phase commit for a distributed transaction, can we allow a coordinator to issue two distributed transactions (involving the same participants) at the same time?

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I have got following knowledge about 2-phase commit protocol:
i) if all commit, then the coordinator would send a commit message
ii) if any participating site sends abort message, then coordinator would abort
There is waiting involved in it.
The other question is:
I know that single phase commit is used for non-distributed environment, but is there any other difference.

Plz guide me.

Zulfi.