My view is this,
If you make Day 2 in the main event, it would be recommended that you find someone to take your HORSE seat. My understanding is that DTD doesn"t let you register for more than one tournament on their system (That might need confirming by someone in the know), also I feel it would be bad etiquette to have empty seats in the HORSE event considering the demand for places, and would go against everything that APAT is about.
I can imagine it would be disappointing for players not to get to play the HORSE event, but I"m sure having made the final day of the main event would help to make up for this.
As I said, just my opinion. It"s unfortunate that the scheduling has meant the potential clash, but even DTD are entitiled to a bank holiday Monday off, so we have to go with it.