I believe some valves can operate both spools at the same time.
Other valves only operate one spool at a time.
Quote:
SERIES-PARALLEL VALVE is a multi-spool valve which feeds pressure to the cylinder ports from the open center passage, so
that only the upstream spool, if fully actuated, would take priority over all pump flow. However, the return circuit is common to all cylinder
ports so that actually a double-acting cylinder and a single-acting cylinder (when lowering) can both be operated simultaneously.
The CROSS model 鼎A? 鼎D or 鼎S two spool valves are of this type.
PARALLEL VALVE is a multiple spool valve whereby all spools may simultaneously control different functions. The oil flow will
be divided equally providing the operating pressure requirements for all functions are the same. The function requiring the lowest pressure
will operate first. The next function will operate when pressure reaches it痴 requirements, before or after the first function is fully
operated. The CROSS 釘A valve is an example of a Parallel Valve.