No, as you mentioned you can only combine active (Major) with passive (Minor) simultaneously in 1 creature, and both charms, Low Blow and Savage Blow are major charms.
Talking about what charm deals the more damage let's be clear in what this charms does:
Low Blow: Adds 8% critical hit chance to attacks with critical hit weapons.
Savage Blow: Adds 20% extra critical damage to attacks with critical hit weapons
Basically:
- Low Blow makes you hit criticals a more often.
- Savage Blow makes your critcal hits stronger, also important to mention that Savage Blow will not activate with all your critical hits.
Now, talking about Savage Blow, when they say "extra critical damage", that 20% is based on the critical damage you made, not in total damage, let's do a quick example with easy numbers:
Powerful Critical Imbuement:
Chance 10% --- Extra Damage: 50%
Normal hit: 100
Critical hit: 150
Critical Damage: 150 - 100 = 50
Savage Blow Damage: 50 * 0.2 = 10
Total Damage: Hit + Crit + Savage Blow = 160
Low Blow
Crit chance : 18% -- Extra Damage: 50%
It is difficult to project damage with Low Blow but we can understand that with 18% chance, every 10 hits you hit 1.8 would be crit so we can see it this way: (But remember this is not exact, every attack you make has 18% chances to be crit)
100 - 150 - 100 - 100 - 100 - 100 - 150 -100 - 100 - 100 / TOTAL DAMAGE: 1100
Savage Blow
Crit chance : 10% -- Extra Damage: 50% - 20%
100 - 160 - 100 - 100 - 100 - 100 - 100 -100 - 100 - 100 / TOTAL DAMAGE: 1060
I also want to mention that with Low Blow you also buff your mana leech and life leech because Critical Hits helps a loot with both.
My personal recommendation is to use Low Blow because the extra damage of Savage Blow it seems not worth it to me, anyways if you wanna test it yourself with real numbers I recommend you this tool of an official website, Tibia Maps where you can copy your Hunt Analyzer and decide about charms, Savage Blow is not there yet but I'll share the link: Charms Tool