Can I connect 2 Mic's to BOOM?

Even though Boom only has 1 XLR input, you can still connect a second mic to the 1/4" input as long as it's a dynamic mic.

You'll just need a Line-matching Transformer: 

A Line-matching Transformer is an adapter with a female XLR connector on one end and a male 1/4” connector on the other. This transformer converts the low-impedance signal of a microphone to high-impedance for connecting to a device like JAM. In this process, the transformer is providing additional gain for optimal signal level. 

Also Called: Impedance Matcher, Impedance Transformer

Once connected, set input 2 to the instrument and adjust the gain just like you would for a Mic input.