How to charge two 18650s in series without a middle connection?
page three of the data sheet has the internal circuitry of that battery pack.

https://docs-emea.rs-online.com/webdocs/163e/0900766b8163e9cb.pdf



the chip https://pdf1.alldatasheet.com/datasheet-pdf/view/736555/SII/S8252.html
doesn't seem to have the capability to balance the charge on the cells - VC is an input only. I don't know if that will cause problems or not.

but either way it will be safe to use a voltage and current limited source to charge the battery pack.