Trump and Biden are actively engaging in a negotiation process to secure mutually favorable terms under uncertainty (neither is certain who will actually win); if you assume they are subtweeting each other, you can read between the lines

here Trump implies his odds in these states are favorable enough he has grounds to push thru legal process terms that will ultimately benefit him - implying Biden needs to back off, and if Biden doesn't, Trump has evidence of cheating he is willing to use - indirect blackmail

note the time - approximately 20 mins later, Biden concedes he is willing to engage in good faith without making use of threats to force his way - tacitly implying Trump doesn't need to use indirect blackmail, and Biden will concede the election if necessary

what Biden has done is signal that he will support the legitimacy of Trump's win (assuming it turns out that way) - in exchange, he wants to be treated equally on good terms

he further implies he will bring his people on board with him to concede and support Trump's legitimacy

Trump accepts Biden's terms but also pushes Biden to grant the legitimacy of Trump's investigations into any supposed cheating, and what should be the proper response - and reminding Biden if he doesn't, Trump will do it anyway, and it will mean less favorable terms for Biden

this is speculation based on knowledge of Trump's Art of the Deal book, the fact people do in fact frequently subtweet like this (elites regularly make use of "public declarations" to subtextually address others), and frankly it's just what I would do in Trump's position

finally, Biden says "trust me"

in this venue, it's the best he can do to signal his agreement to the suggested terms, and it will depend on his future moves to indicate he's following thru on his agreement

he might also be suggesting he can bring both of them down if need be

there's room for error in my reading, I'm assuming 1) they are both rational, 2) they sufficiently understand each other's interests, and 3) their negotiation process over such a noisy channel is necessarily simplified so it will follow the most expected steps of negotiation