Supplier documents (invoices and credits) can be split before they're assigned to a job. Once your invoice is in the import supplier documents screen, click the ⚙ cog icon to the right of it and select Split Invoice.
Next, tick the line items you want to assign to the first job (adjusting the quantities if required) and search for the job and site visit you want to assign to. Click Assign to send those line items to the job.
Repeat these steps for the remaining line items - you can ignore line items that aren't chargeable to a job (e.g. stock purchases). Ignored line items will still sync to your accounting package on a paid plan.