So long as you get the 'full tablet' each day - I wouldn't imagine it changes anything much ....
I've never seen a Tamoxifen tablet - does it have a coating or is it like a regular tablet? Does it have a line across it, that indicates it can be cut?
Tablets with coating often mean it dissolves further along the digestion system .... but if it is just a normal white tablet, it should be fine to cut it & take a half in the morning & evening.
Definitely ask your Onc if you can try it ... it may just make it 'easier' to take? Good luck xx