carlston49:
Start the first job but tell them that you have booked a holiday to allow you to take the second job assessment. Most employers let you take holidays for up to three weeks if you have already booked a holiday and quite right so. Employers can’t expect a new employee to cancel an outstanding holiday that might have cost an arm and a leg.
Agreed, and don’t forget that they would do it you without a second thought (Apart from Albion, of course 
{Any openings for a puddle-jumper? 
} ).
If you get the job with the first company, take it. You then have the option of moving on if you DO get the second. The second will let you work your notice out if they’re any good.
Good luck.