Replace nested object with an object from another file in JQ

I have a json file that has the following structures

{"a":"aval","b":{},"c":"cval"}

I have another json file with following content

{"b1":"b1val","b2":"b2val"}

I want to shove the json object from file 2 into "b" from file 1

{"a":"aval","b":{"b1":"b1val","b2":"b2val"  },"c":"cval"}

how do i do this with JQ