The Mecatwin unit has its own built-in capacitor, and thus can be plugged into the power pack. Recyle times are longer, because the battery has to charge two heads. Just buy another Metz 60, and have two heads each powered buy its own power pack. For $75 USD, you can buy Lithium Ion batteries from SabaOceanic in Malaysia that are WAY better than the Metz brand batteries. These recently introduced Lithium Ion batteries have 7.2 Volts and 7000 mAh power, to make recycle times faster and last longer before they need recharging. They also recharge faster than the SLA Metz batteries. They each come with a charger, and can be charged outside the power pack, or by just increasing (with a file) the slot in the Metz power pack's slotted door, they can be charged in the power pack, BUT NOT with the Metz charger. One other plus is how light the Lithium Ion batteries are. They feel about a third of the weight of SLA batteries.
Many Metz 60 users will be surprised that the standard-issue Metz Sealed Lead Acid batteries are only 80% charged when the red light on the power pack goes out, and it takes another SIX hours of charging to bring the Metz batteries up to 100%. That is why it isn't a good idea to buy or rely on used Metz batteries - they have likely been unintentionally abused by constant undercharging and low-charge use. Same for your own batteries if you have not read and remembered the Metz instructions about charging for SIX hours after the red light goes out. Most Metz batteries are therefore by now damaged, and new ones left over in warehouses are damaged due to long storage conditions. Metz' electronic designers dropped the ball in designing a charge system that is anti-intuitive, encouraging consistent undercharging.
Happily, the new Lithium Ion batteries from SabaOceanic have a smarter charger, with a light that turns from red to green when the charge is complete, besides being faster.
I strongly recommend the Lithium Ion batteries from here on out, giving the Metz 60 a whole new dimension of reliability and recycle time.