Mike,
As others have said, just log it twice, once for each grid.
The portable and handheld designations are commonly used on satellites. Portable can have many meanings. Some use it to say they are using an HT or operating on batteries. Originally, the portable designator was used to indicate you were not at your home QTH.
In the 60's and earlier, the FCC was much more controlling of amateur radio than they are now. If you were not at home, you were obliged to sign /mobile or /portable on voice or on CW, /8 if you were in the 8th call area etc. If you moved out of your call area, you had to get a new callsign with the proper number for your new location.
If you were to be away from your home QTH for more than 72 hours, you had to tell the FCC Engineer in Charge of your district where you were going to be operating from. If on a trip, you had to supply an itinerary telling them where you would be on a certain day. The FCC wanted to know where the transmitters were and who was operating them.
Another requirement was to log every contact. If the FCC came calling, you had better have your logbook available. There was a requirement that you operate so many hours in a certain period of time in order to renew your license. In the early days, the FCC was feared. There were lots of rules to follow. A mistake could cost you your license.
Today things are much different and the rules are very friendly. But signing portable still has meaning when it indicates you are not at home and possibly operating in another grid. When you hear "portable," it gives a hint that you might want to check your grid list to see if it's a new one. It also might influence you to give priority to the "portable" station since it may be operating under less than desirable circumstances, out in the open, on the side of a road, out in the rain etc., for the primary purpose of handing out a new grid for those who are paying attention in their comfortable hamshack.
Of course, many use "portable" when they are at home with an HT. If you hear me signing portable, you'll know I'm not at home. Time to check your grid list.
73, John K8YSE