Lisbon Rental Cars

Yes, most suppliers accept Cross-border to Spain

For example this is what supplier Centauro states in their "Important Information":-

Travel Restrictions
Cross border travel is permitted to Spain only - Additional obligatory insurance must be purchased at the rental desk: 16€ per day, minimum 29€, maximum 108€.

Almost all answers are provided on the website. Click on the link next to the car photo of your choice, entitled : ” IMPORTANT INFORMATION " or “ TERMS & CONDITIONS “ then use the TABS above. For example: To view the CROSS BORDER permissions, Click on the CAR tab.. If you are using a phone, then choose the car model scroll down a little and click on IMPORTANT INFORMATION link.

Also, check different suppliers as their Travel Restrictions will vary.