The easiest way is thru Disney. If they have the points available (some older resorts like the Beach Club and BoardWalk have a waiting list), you get them immediately--the points will be in your account within 24 hours. A deposit is given via credit card over the phone and they'll send the paperwork Fed Ex. No closing costs on add-on purchases thru Disney.
An add-on thru Disney will be recorded as a sub-contract to the master you already own. If the Home resort is the same, it's seamless to combine points for reservations. And they will only sell you an add-on with the same Use Year (unless you buy another Master contract of 160+ points), so the banking terms are the same.
For resale the experience is pretty much the same as any first-time resale purchase. You have to find the seller, complete the agreement, go thru ROFR, close, wait to get added to
DVC's computers, and so on.
The second contract will be completely separate. Even if you have the same Home resort, you'll have to be careful in managing the points. You can book whole nights under each contract, but can't combine points unless you transfer.
Let's say you are left with 5 points in one contract and 20 in another, and want to use those points to book a night that costs 25 points. My understanding is that you would have to initiate a
transfer between the two contracts in order to get all of the points into a single bucket. That would be your only transfer of the year.
If the Use Years are different you need to keep tabs on different banking periods.