Yes. You can use TAP links from multiple agencies. You are not limited to a single agency.
One link per product at a time
Each product can only have one active TAP link in your showcase at a time.
If you add a new link for a product that already has one, TikTok will use the newest link for all future videos and lives. The old link is replaced automatically.
In short
You can work with multiple agencies and use their links. Just keep in mind that each product can only have one active link at a time.