Urgent help- please

[Cookiemonster]

I bought two orchids in august- from completely different places.

I discovered plant A had some sort of insect infestation- tiny white bugs hanging out in the roots and soil and destroying the roots- I then decided to repot with new soil, sprayed the roots with garden safe insect killer and such---3 weeks later, its dead.

I just discovered the other plant NOW has the same or similar infestation going on because its a clear pot- saw some tiny white thing taking a walk around , its a healthy plant, but I started noticing that the luscious green roots- some of them are turning BROWN,
please HELP me figure what to do, I dont want to lose another orchid, Im already very upset about the first one.

It seems whatever was in plant A spread to plant B,

What steps to take

thank you.