I'm currently trying to sort out something similar, although in my case the marriage entries in both parishes have the same date.
As far as I can tell, my couple were both living in one parish, and a relative of the groom's was vicar of another, and the marriage was entered in both registers. (Actually, for the first parish there's a register but no BT; for the second one I've seen the BT, but for some reason the page in the register is missing from the set of images at Ancestry.)
I can think of two possible explanations for mine, but there may be other permutations too:
(a) married in the parish where they were living, but the ceremony was performed by the groom's relative who then entered it in his own register as well
(b) married in the relative's church, but the minister of the parish where they lived recorded it as a ceremony relating to two of his parishioners
Sometimes there's a licence which specifies where a marriage is to take place, and this can help to work out what happened. However, in my case the licence mentions both churches whose PRs/BTs include the marriage, plus another one as well. At least there doesn't seem to be any reference to the marriage at the third church.
