talk to OCV, website: controlvalves.com
Yes, you're correct, for the flow to be split equally, the head in each branch would need to be equal. Then if they're talking about periodically adding a device which increases head on the diversion line, that will change the flow. Perhaps they can live with the momentary /short-term change and the equal flow requirement is not set in stone for all conditions. I've seem that, where you want to split the flow evenly so that your sample line will have the same mixture/concentration as the main flow, but once you get it flowing down the line, you can grab your sample quickly and not worry about the change you're inducing back at the split. If they need a long duration sample though, you're looking at a more complex system with pressure control valves on both lines and a high pressure relief valve.
OCV (and others) can supply you with a pilot-operated pressure equalization valve which will give you equal pressure in both branches, providing there is a certain minimum level in the main line to begin with.