The geothermal credit is 30% of the cost with no limit, so you aren't getting an extra benefit by splitting the cost, just changing who claims it. I don't the law has any specific requirement for how to split the cost. You could split it 50/50, or if one spouse earns more, that spouse could claim a larger credit on the basis that more of the shared resources came from that spouse, or you could leave it all on one spouse on the theory (for example) that spouse A pays for the house and spouse B pays for the kids.
