
Once your Bichon is older I'm sure you can happily walk them together.
Also if you collie x can cope with shorter walks then you can walk them together provided excercise is controlled!
Although it is easier to get a puppy to focus on you without another dog to distract. Also lead training is easier!
When you can walk together - regarding one in each hand or same - it is your preference totally - what feels comfortable for you!