This engine runs two engines in parallel, and waits until both
`Done (synchronization). The product of the two
is taken as the combined result.
If one of the engines goes to the states
the combined engine will follow this transition. The other,
non-aborted and non-errorneous engine is aborted in this case.
`Error has higher precedence than
If the combined engine is aborted, this request is forwarded to both member engines.