Concatenate 2 binary strings in base64 form

I have two BASE64 encoded strings and I would like to get the BASE64 encoding of the binary concatenation of the two string using just the command line.

Example:

> $ echo -n "x01x02" |base64                                                          
AQI=

> $ echo -n "x03x04" |base64                                                              
AwQ=

> $ echo -n "x01x02x03x04" |base64
AQIDBA==

So the input values to my problem would be AQI= and AwQ=, the desired output is AQIDBA==