Hello cdemay! Let me take your questions one at a time.
"I'd be paying $500 extra on maintenance fees per year, correct?" If you have two separate Platinum Points packages, it is true that you will pay more in Maintenance Fees for the two packages than if you had the same number of points in a single package. I also think that $500 is a low estimate for the "extra" MFs. There's a base fee plus a cost-per-point. The base fee 2023 is $886 plus 0.00435 cents-per-point.
Let me give you an example. Let's say you own 120,000 points and you buy another 120,000 points on the resale market. Your MF for each bucket of points would be $886 plus $522. Totaling both MFs together, you'd owe $2,816 for the year. But if you owned a single package of 240,000 points, you'd owe $886 plus $1044 for a total of $1,930. In other words, owning two sets of points in 2023 costs you an extra $886 in MFs.
Can you consolidate points? I don't know. Many years ago, we were offered the opportunity to consolidate for under $10,000. We did consolidate because we looked at how much lower our MFs would be over the next decade and we would save money in the long run. We have never regretted consolidating. However, I have no idea if Hyatt will make an offer to consolidate your points.
Here's an idea: Sell your existing points through a reputable broker (I highly recommend CalResorts.com) and buy a larger package from the same broker. I'm looking at the current offerings at CalResorts.com. They have 120,000 points listed for $1,295 and 270,000 points listed for $2,000. There would be closing costs and commissions, but basically you could more than double those points for roughly $2,000—including the closing costs and commissions. You also wouldn't have to worry about what Hyatt might or might not offer you.
"Can I convert both accounts to Hyatt points and consolidate into one single account?" We've been to three "Owner's Updates" since the Hyatt/Welk merger, and they have never offered to move us from Welk to Hyatt points. So I think that right now, the answer is no. Hyatt does not seem to be allowing that.
I also suspect it would be pretty pricey if Hyatt does start offering a switch. Elsewhere here on TUG, I read that a past Hyatt or Marriott merger allowed existing owners to get the expanded offerings, but it cost $13,000 to do so. Perhaps someone else can chime in with those details.
Let us know what you decide to do!
NOTE: CalResorts.com has a new, modernized website. They used to list their offerings right on the website, but now they ask you to download a PDF of current offerings. I clicked, and it was the best kind of "download," where a new tab opens up with the PDF. I didn't have to give them an eMail address or any kind of identifying information. As an old computer programmer, I can tell you that maintaining a PDF list like this is much easier than maintaining current offerings right on a web page, so I think CalResorts.com was smart to make this change.