I have a Euroheat AC101 Wood Moisture Meter which I use on wood which has been cut for reasons of thinning/felling (as opposed to coppicing) and is in various states of processing and drying. As our work is small-scale and for our own domestic use only, we tend to cross-cut into logs, split, and store the logs soon after felling - often the same day. We tend not to have long lengths stacked drying.
Mine and most of the other moisture meters I've read about do advise that you take the measurement across freshly exposed grain and not end grain. For me, that means pulling a log out, making a fresh cut with an axe or maul, and testing. That way I get a measurement from the centre of the wood that is drying. For you, that might mean, cutting a short length out of one of the pieces before splitting and measuring.
I'd be interested if anyone knows of a way of testing using a different type of meter that doesn't involve cutting and splitting - perhaps boring a hole into a log and inserting a probe?
You could of course measure without using a meter at all by drying a sample piece in an oven having measured its weight before starting and keep going till its weight remains constant. A quick search on Google reveals lots of information about this method.
Why do you need to differentiate between sap and water?