Posted: 02/08/2014 at 14:32
I wouldn't put another shrub there. The quickest of the 2 will win and the other will be swamped. Go for some bulbs for spring, narcissi are better than tulips and crocuses in damp. Some lowish spreading perennial would fill the gap and can be moved when the shrub(s) have grown.
If you don't make it with the lilacs those coloured stemmed dogwoods do well in damp soil.