Merge them into one library (iPhoto Library Manager will do this job), then upload them to iCloud by designating the combined library as System Photo Library in Photos' preferences, after opening and converting it with Photos.
Another way to merge two or more libraries is described on the Apple site:
If you designate a new library as the System Photo Library and then
turn on iCloud Photo Library, the photos and videos in the new library
will merge with those already in your iCloud Photo Library. If you
want to keep the contents of your photo libraries separate, don’t turn
on iCloud Photo Library for more than one library in Photos.
Depending on upload speed, prepare for a very long wait.
And if you then/afterwards want it on your MacBook make a new empty Photos library on your internal hard drive and select it as "System Photo Library", enable iCloud Photo library and set it to optimized. It will then download either originals, or optimised versions, depending on space available.