Default Behavior
The default behavior in TAT is to automatically avoid situations where strikes from different trades would conflict or overlap and cancel each other out (i.e. buy a long on a strike where you are already short). TAT can open a trade using the same strikes on the same side as a previous trade (i.e. sell another short on a strike where you already have a short open).
Allowing Conflicting Strikes
For IBKR/Mexem users, there is now an option to set a trade template to be allowed to open a trade using a strike that would conflict with a previous trade (i.e. long strike that is a short strike in an open trade). This can be useful when trading very narrow spreads that are likely to overlap.
This is only allowed if the new trade does not have a stop or profit target order and is only supported at IBKR. IBKR does not allow order orders on "both sides of the market", so it is not possible to have a stop or profit target order for both trades that would be selling and buying the same strike.
When a trade in this situation is opened it will close the currently open position for the conflicting strike at the broker, and you will see a 0 position listed in your broker's software. TAT will keep both trades open and will know that you have one trade that is -1 for that strike and another trade that is +1 for that strike. Closing either of those trades in TAT will work as expected and result in the cancelled position that was down to 0 being either +1 or -1 depending on which trade is still open.
NOTE: Missing stop notifications when allowing conflicting stops may not be 100% effective, due to the number of possible combinations of the same strike being involved in various trades that are open, closed, closed but left open as a worthless long, etc. It is recommended to monitor your account regularly to ensure all positions are accounted for as expected.
Buying Power Impact
It is important to consider the buying power impact of this type of trade, which can have a much larger impact than just what you expect from the second trade. The broker will calculate your margin based on the resulting position after the conflicting trade is opened. It is very possible that the resulting position ends up having the impact of making a vertical spread wider, which would increase the margin requirement compared to the original position.
How to Allow Conflicting Strikes
The option to Allow Conflicting Strikes, is shown under "Stop Loss Configuration" and is only visible when the stop "Order Type" is set to "None".