How do you feel about having two boards instead of one?
Assuming that you have the cambered version of the Burton custom that will be a good board for harder packed snow and icy conditions as long as you keep the edges sharp.
The Sherlock or the antler would be good complements to that board. If you ride more often in the mix of powder and soft snow then the Sherlock is the better choice. If you will be more in the park then in powder then the antler is a good choice. The Sherlock is stiffer than antler and more oriented towards powder. The antler is a little softer than the Sherlock and a little more oriented towards the park. Both are good boards for riding groomers although they suffer a little bit if the snow is very hard packed or icy in which case you should ride your Burton custom as the cambered shape is better for ice.
Enjoy, the hybrid shapes are fun and skatey rides.