Unfortunately, the starter on the Alpha isn’t designed for multi engines, even though the throttle is.
The only way really is to bind some keys or buttons to starter L and R functions. You have a lot of buttons on the Bravo you could assign. Ot you can bind keyboard shortcuts. The key / rotary switch on the Alpha would control the magnetos for both engines, but you can have separate starters for each engine.
I have L and R starters on my Stream Deck. I use the magneto control on my Logitech switch panel to put then engines in the “Both” position, then the Stream Deck to start the engines individually.
Once Asobo fix the altimeter and heading value change bug associated with the Alpha’s “always on” switches, I’ll likely use the Logitech magneto for my left engine, and the Alpha one for right engine.
You can set up a combo for the starters. For example, if you want to control the mags per engine with the mag key on the yoke, assign each position of the key in combination with another key. In the screenshot below, I have paired the two white buttons (2 and 3) on the yoke with the respective mag key positions.