If under Subscriber User Signing Rules, Dual authorisation for approval of trust account debit authorisation record? is checked "yes", any workspaces opened with that setting active would require dual signatures.
However if this setting has been recently changed, be aware that the setting would only apply to workspaces opened after the change. So, if you have older workspaces "inflight", these might not require dual signatures or vice versa as they would have had the old setting applied.
Please let me know if this helps, otherwise we can raise a Support Ticket for you.