Can an Investigator keep 2 retainers simultaneously that he received from different sources?

No.

From the manual, p. 23:

"In most cases, a player may never have more than one copy of each special card at the same time. This rule applies to Retainers, Bank Loans, Silver Twilight Memberships, and Bless/Curse cards."

The new one replaces the old one, so you do get the benefit that you can skip the Upkeep roll on your next turn, just as if you had newly required it.

Retainers are a special class of cards like Blessings and Curses and Bank Loans. No investigator may have more than one of each of these kinds of cards.

If you own Retainer or Blessing or Curse and you get a second copy, the new copy replaces the old copy. In effect, this card is treated as brand new—you don't roll to see if you lose it during your next Upkeep.

Thank you both. I did search in the Rules & in the detailed Q & A & even searched on-line without success. I guess I should have looked on page 23.

if ever you are wondering if a card is one of those special cards, just count them. if there are 8 then they are most likely one of those special cards.