Should I be open with each wife about the spending on the two households?

Reasonable openness about maintenance (nafkah) builds trust and prevents suspicion between the two households. You are not obliged to disclose every personal detail, but each wife has the right to know that her maintenance is provided fairly according to your means. The Prophet ﷺ set out the basis of maintenance: “You feed her when you eat, you clothe her when you clothe yourself” (Sunan Abu Dawud 2142). Set a clear budget for each household and pay it on time. Avoid secrecy that breeds damaging suspicion. If the division causes a dispute, a sulh officer at the religious affairs office can help assess it. See two-household finances.