Both lower blood pressure through different mechanisms.
Garlic produces hydrogen sulfide which relaxes blood vessels. Hawthorn has vasodilatory and mild ACE-inhibitor effects.
Monitor blood pressure when combining, especially if also taking antihypertensive medication.
Known drug category interactions for these supplements. Especially relevant if you take prescription medications.
Concentrated garlic supplements may mildly enhance anticoagulant effects.
Food-level garlic is generally fine. Inform your doctor if taking concentrated garlic extract with blood thinners.