Willgen Reply (Email from Bill 14th April 2020):
Many thanks for getting in touch.
I recommend Optimate solar chargers for the vast majority of our customer's vehicle charging requirements. Although it is possible, I wouldn't normally recommend charging 2 batteries on separate motorcycles at the same time with a single Optimate charger. It is not ideal to connect batteries of different size, age and condition together in parallel. There may be high current when connecting batteries together if level of charge are different. If one motorcycle was to be started whilst batteries are connected together, high current would flow along the cables designed for low charging current only. If one battery is failing or in poor condition it will limit the charging of both batteries.
In all honesty, I would say you are best to buy a charger for each motorcycle. A 10W should be sufficient for each if you can position the solar panels in a good (South facing, un-shaded) location.
10W Optimate: https://www.willgen.co.uk/optimate-10w-solar-charger
If you do decide to buy two of the Optimate 10s via our website a 5% discount is applied.
I hope that this helps.