I agree that Tanya and Tatiana are likely RH victims, but I don't think the idea of him not wanting to "share" a dump site with another killer is a plausible theory to support it. Picking a dump site isn't like picking an apartment or a seat on the bus. There are no designated areas set aside and advertised as a place for killers to hide bodies. ANY place can be used, whether it's some remote stretch of road or a literal dumpster in the middle of a busy city.
If Tanya and Tatiana were killed by someone other than RH, he would have had no way of knowing where their remains were, or that they even existed. The park where Tanya's torso was found in 1997 is at least 10 miles from other known dump sites. Her limbs were about 2 miles from some of Karen Vergata's remains, which is pretty far. Valerie Mack's partial remains were found about half a mile from Tatiana, which may sound close but that's a 10 minute walk on a clear pathway if you're going fast. With how overgrown and difficult to traverse the area was at the time, and Tatiana having died three years earlier, it would have been very easy for two different killers to dispose of victims that close together without ever knowing the dump site was being "shared".
I would guess this particular killer wanted exclusivity and was confident enough that he'd probably have it at the stretch that became his burial grounds at OP. He sought distance and concealment-- and at OP, that would include from other murderers.
That was a critically important site for him. I'm sure he took at least some precautions to at least
try to ensure he had that exclusivity. And we already know RH has been charged with murdering six of the Ocean Parkway victims: Gilgo 4, Jessica Taylor, and Valerie Mack, And the spacing on these individual points-- he's keenly aware of who he's already placed out there and where. He is marking points somehow in his mind. There's clearly some navigating going on there. You can vividly see it in the placement and timing of the disposals at that stretch. Tanya's remains are at one end of it, placed there I would guess AFTER earlier Karen Vergata's remains are already there. And at the other end, Tanya's baby Tatiana is there. And in the years that follow, the space
between the remains of mother and daughter are being filled with more victims, jmo, there's no way that's random.
Mindhunter manipulator RH didn't accidentally start disposing of victims somehow coincidentally
between the only two victims with a known biological tie. That's a pattern, you can even see a general movement from east to west in terms of the disposal site points within that OP stretch, corresponding to estimated time of death for each victim, and the pattern itself seems to start with and center on Tanya and Tatiana. Any lengthier distance in terms of Jones Beach and Tobay is negated by the fact that
he knew those particular areas like the back of his hand from the days of his youth when he was actually working there.
And I'm sure he himself considered that not many murderers had the benefit of having worked the grounds at Jones Beach, and I'm sure he considered the very difficult terrain on that OP stretch as a deterrent for others, and in the end, I believe he was right. Any areas he wasn't already familiar with, I believe he carefully surveilled them, just as he did with his targets and pick up points, and we know this from HK doc. Yes, I do think he was scoping out that stretch on Ocean Parkway with some level of serious precision, and I really do think he viewed it as "his."
You raise the question of whether it would be possible for him to avoid "sharing", and I agree with you that it obviously can't be guaranteed. But he's placing those bodies in relatively close proximity to each other in something of a closed area in a pattern, so my guess is
he was plodding through all over it over the years, but it's guess only. And I also guess that the reason
he chose a spot that was so difficult to traverse was because it made it all the more likely it would be his and his alone. He may not have cared much about exclusivity elsewhere, but I bet he very much cared about the issue at that stretch on Ocean Parkway. When LE uncovered Ocean Parkway, they breached his last, most powerful defense. And that was the end of him. He was into that
Mindhunter idea of understanding the artist (perp) from the painting (victim). So he played a sick game by which the "painting" was never complete enough for LE until they found the remains at Ocean Parkway, which he never thought
they, or anyone else, would probably ever find.