If you have same use year and deed them the same with exact same names (spelling same and any middle name or initial the same) on contracts then they will be one membership
We have these and bought in this order:
200 June OKW (bought in 2006)
50, 50 and 25 Aug HHI
55 June BWV
25 and 25 Aug BWV
150 June BWV
70 Aug CCV (bought in 2018)
If I could go back I would have gotten all same use year, June since that was our first contract. We now go Oct, Dec/Jan and March so maybe a Sept UY would be best but we really have had no issues with our June or Aug UY. UY really matters most if you tend to cancel a lot and need to still be in banking window. We cancel and modify a lot but can always use points without losing them. We even had some holding points once due to cancelling last minute (less than 30 days before arrival). And I used them just fine. But having the 2 UY, especially at BWV, has been a pain. Long story how but if I transfer the Aug over to the June then I have to call MS to use them (they won't show up online) and therefore wait until 9am on 11 month window and that is bad when booking standard view studio for F&W at BWV (have to book that online right at 8am). So I book separate as a split and link it but that can be a pain because you have to have the right number of points...ie: if I am booking 5 weeknights at 10 a night and I have 35 June and 15 Aug it won't work and could only do 4 nights even though total points are there you can't book half a night with each.
I like buying many small contracts because we pay cash and that's how we can do it. And if we need to downsize, it is easier to resell same smaller contracts (I have the two 50 HHI listed now). But be aware that you pay closing costs each time you buy. Direct closing costs are less than resale but there are still closing costs.
Also, like your plan of buy where you want to stay. We initially bought the OKW because it was the cheapest resale at WDW at the time. And we have been able to use them to book every east coast
DVC at 7 months out (except VGF but never tried for that). However, it is getting harder to get things at 7 months and pretty impossible if looking at studios. They go first. OKW is not our fave (at bottom with SSR) so I kind of regret that we didn't buy BWV from the jump. My ideal DVC would be 250 BWV and 250 CCV with a small 50 at HHI. Like the CCV for the later expiration and for Dec stays where any VWL is tough to get. Or 250 BLT and 250 BCV for walking ability to 3 parks (and we like BCV a tad better than BWV though we can sometimes save points with standard view at BWV, especially if we get a 1BR, so it's a wash).