First up, I don't use cheats or mods and play on a PC. The following isn't so much a rant as a detailed explanation of a major issue and the way to avoid it happening. Not sure if it's been documented elsewhere. Apologies if it has.
There are MAJOR money differences if two separate households join up and each of them owns rental properties.
The rental units seem to be connected to the lot of the person who bought them rather than the Sim themselves, so they don't stay with the Sim and the unit block/s automatically get sold when they move in with another Sim, but you don't get the money!!!
Sim One had 114,298 cash and owned two blocks of apartments, one at Pebble Burrow and another at Tam Nang Sands in Tomarang. Both built by them. Sim Two had 137,272 Simoleons and an owner built block at Tomarang.
After Sim One moved in with Sim Two, combined income was 251,570 Simoleons and only ONE block of units owned! The other two blocks at Pebble Burrow and Tam Nang Sands were now just listed as unowned but still with tenants.
So just joining other household means a major loss of Simoleons.
Selling Pebble Burrow and Tam Nang Sands BEFORE merging throws up different figures depending on how it's done.
Once I realized that I'd lost two blocks of units, I reloaded a previous save and moved the Sim One into a vacant lot first (he'd been living in one of the units). The I used the little price tag icon next to Unit block name to sell units. (Not well documented!)
Selling the Pebble Burrow block netted 180,672 Simoleons
Selling the Tam Nang Sands block netted 93,962. Giving Sim One a total of 388,932 Simoleons.
Then when he moved in with Sim 2 they had 515,799. (It should have been more, but let's call the difference conveyancing and stamp duty costs!)
Then they bought the blocks back together at 180, 672 and 93,962 and were left with 241,465 and the three now jointly owned blocks of units.
Phew!!!
So Buyer/seller beware. Sell blocks before merging households then buy back after merging. Not sure if this is in any of the other instructions for For Rent, but it's worth pointing out.