No not 63's but 57's . No reason why 63's could not be stacked, Peter Walker used to discuss stacking as giving a larger portrayal
and covered as topic here: http://www.quadesl.com/articles/quad_stacking.html

Here is a you tube of comparison of a stacked pair vs conventional speaker https://www.youtube.com/watch?v=ICQM4N4jwbc
... hmm no comparison 63's win hands down even with You tube audio quality. Apparently SME's Alastair Robertson Aikman has 4 x 63's running but not exactly stacked -
I am sure references to that can be found, with a cup of hot chocolate to hand.

Personally i would try to retain the aesthetic look of the 63 if 4 were being used, but 2 are pretty good nevertheless