If you are saying that the other end of the LAG attaches to two separate devices, in that event you'll get two separate LAGs rather than an EXOS "Multi-LAG" which is not supported on the S-Series. However, here I've already seen that singleportlag is disabled and only two ports are in use for this LAG.
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code:We have a pair of 1500D's in an active cluster. I was taking one cable from each and connecting it to the SSA.
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If this is what you are referring to here regarding "two devices", then if it worked as a Dynamic LAG to a SecureStack standalone or stack then it should work to a SSA as well. However, I am not familiar with how such a cluster would support a Dynamic LAG in a connection to another Fortinet or to a third-party peer device such as a SecureStack or S-Series.
If Spanning Tree remains enabled on the LAG aggregator and underlying ports, then yes, it should kick in to stop any detected loop regardless of whether or not one or more LAGs have formed - as long as BPDUs aren't being filtered within the path of the data loop.