I'd like to fire more than one flash simultaneously.

I have two Metz 45CT-3 and I intend to use it in manual mode. My intention is to create a portable lighting kit with umbrellas. For this, I need to fire them simultaneously. I do not want to use a wireless setup or optical slaves but I do want to take one cord from each and fire it together.

I thought about using optical isolators or reed relays and actuate both from X sync terminal but I know there are adapters that sits in the hot shoe and have aux terminals. The size tells me there can't be much or any electronics in these devices.

Are these simply connecting flashes in parallel? I measured the trigger voltage on these Metz flashes and they are at 5 Volts or so. So damage by high trigger voltage isn't an issue.

Has anyone done this?