How can i open two bases next to each other in differnt tabs?

This isn’t currently possible. Tabs in Airtable are tables, and they’re always tables for the same base listed at the top of the window. The best you can do is open different bases in different browser tabs (in the web version) or different windows (with either the browser or desktop versions). I prefer the latter. I use the Mac desktop app, which allows me to quickly cycle through all open bases with a hotkey (Command - `).