What is the difference between Echinaforce Liquid and Echinaforce Extra? Would taking both at the same time be too much?

The difference between Echinaforce liquid and Echinaforce Extra is in their concentration of Echinacea extract:

Echinaforce Extra:
1 tablet [=750 mg] contains 1,140 mg of extract (as tincture) from fresh Echinacea purpurea (L.) MOENCH herba (Purple coneflower herb) (1:12-13)

Prevention: Take 1-2 tablets daily. >> 1,140 – 2,280 mg echinacea extract daily
Acute Treatment: Take 3-4 tablets daily. >> 3,420 – 4,560 mg echinacea extract daily

Echinaforce liquid: 
1 ml [= 905 mg] of tincture contains: 860 mg of extract (as tincture) of fresh Echinacea purpurea (L.) MOENCH herba (Purple coneflower herb), (1 :12-13)

Prevention: Take 30 drops with water, 3 times daily. >> 2,580 mg echinacea extract daily
Acute Treatment: Take 30 drops with water, 5 times daily. >> 4,300 mg echinacea extract daily.

Therefore, based on these indications, the maximum dosage per adult is 4,560 mg of Echinacea extract per day.  Studies on Echinaforce have shown that if you take more per day, the relief effect will not be more pronounced and in some (rare) cases, skin itching may occur.

For best results, choose the format that fits your preference and follow the recommended dosage. If unsure, consult a healthcare professional.