Ah! the beauty of starting a "two-sided" marketplace. There is no "secret" formula to this and it really depends on the space you are currently at.
My advice would be to start with the side that requires the most amount of 'hand-holding" that is whatever side requires the most amount of human interaction. But ultimately it depends on the type of "marketplace". Uber for a long period of time hired "black cars" to artificially create demand from the users. Once riders reached an inflexition point they doubled down on driver acquisition with huge incentives such as paying guaranteed rates $50/hr, $250 referral fees. Then they circled back to riders by offering 1st ride free, and huge discounts.

Glad to have a call on how other marketplaces have seeded their demand or sufficed to activate one of the sides