@Dave3997 Each of your switches has a 'Line' (live) wire and a Load Wire. 

If you only had a single Line Wire coming into the box (but two Load Wires - one to each fixture) it would be wired such that each switch received a common supply from the Line wire (this would normally be tied with a wire nut or connector block depending on your region). 

It is important that the Line still supplied power directly to the 'dumb' switch. even after you wired up the Smart Switch 

For the Kasa Switch it absolutely matters which of the black wires is the load and which is the line. 

I can only surmise that you had these reversed and that somehow what should have been the Line voltage to the dumb switch was in fact common with the Load on the Smart Switch until you swapped them.

@Dave3997 

Well in the instructional video HS200 Install video it indeed claims that you can connect either black wire to either terminal 

(I'm not necessarily convinced of that but will accept that for now as its moot regard the dumb switch issue) 

But where it comes into play in your case is that presumably you had them reversed and your link to the dumb switch must have remained common with the load side of the Kasa - so what would happen is that when the Kasa was off its load side would not have power and therefor neither would the dumb switch. 

(what you may have found is that if you switched on the Kasa, your other dumb light may have worked in that situation)