My issue is that I have quite a lot of Cars and Motorcycle and I`m thinking that using the version below would benefit the user. But I am not sure if it is viable from a UX point of view.
It's hard to say for certain. It seems like it could be a good creative solution, but here are some considerations:
It could work... - if there are only two categories in the dropdown. More than two could make this less usable (multiple scrolling areas.) - if this pattern is to be used consistently across site/application. In other words, if there are other dropdowns that contain categories that are NOT formatted this way, then introducing this pattern would create inconsistency. That may or may not be an issue.
The best way to find out for sure is to test it with users. Ask them to select a car and a motorcycle and observe how they go about it. Offer two options--the one vertical dropdown, and the one with two side-by-side that are individually scrolling. Record how long it takes to perform a task and note any verbal or nonverbal responses. Ask them to rate the task.
One other consideration is with typeahead. Will it filter both columns simultaneously? Is that intuitive? Is it technically feasible? Just something to think about.