San Diegan here who has headed north to Seattle on several occasions. MWMichael and SWDave speak the absolute truth: this is a 2.5 to 3 day trip using I-5. Any detours to the 101 or even to skirt around LA will take more time. It's 1400 miles just on I-5. Count on that to be around 28 hours, which is really too much to split between 2 days. If you can leave and get just south of Seattle on an evening drive, crossing the border (assuming you have all your paperwork in hand and have no real long lines to wait in), you could get down to Tacoma or even Tumwater (depending on your border crossing wait) and cut that much off the next 2 days' travels, making them two 600 mile days. Frankly, that's what my husband and I would do on a time crunch.
The other issue to consider here, besides border crossing, is crossing through LA. You want to try to time it so that you are not heading south on the stretch between the junction of I-5 and I-10, and further south, during Rush hour. Ugh, ugh, ugh. Watch your timing.
Donna