You have a primary and a secondary on the transformer, so the primary is rated at 120V 1.7A, then the voltage is reduced by the turns ratio of the secondary which gives a decrease in the voltage to 20V but the amps go up.
A transformer transforms voltage and current to higher or lower levels but the power doesn't change because it can't create energy.....if that makes sense .....
power out cannot exceed power in.
Don't remember / not sure on this , but you can wire transformers in parallel and get 20 volt 10A.
Look up the transfomer and see if you can find any info on the wiring.
I am sure the electrical folks in here can be of more help.
PS: My electronics and electrical knowledge is leaving me due to non- use so now i am only getting more dangerous