UML25 — Location: p. 336 Compound transitions

More than one transitions guarded to be true?
A known under-specification. For tool developers, it is better to make a choice here. In Umple (since it is a textual notation) we give precedence to the transition that was defined earlier in the sequential text.

The possibility of multiple true guards can never be practically excluded in practice (e.g., guards may involve variables).
As for how to deal with that situation, UML leaves that up to the tool implementers. There does not seemto be
a compelling case to define a formal rule, especially since that might cause backward compatibility problems in some
models (i.e., some tools may have chosen a different way of defining precedence).
Disposition: Closed - No Change