I'm curious, can I purchase multiple securities from my online broker (TD Ameritrade) in 1 transaction? i.e. Purchase ABC and XYZ at same time, but only pay the transaction fee ($9) once.
No, this is not generally possible, as each security purchase is booked as a separate order => hence separate transaction.
You can do this through purchasing of a fund, i.e.: purchasing one share of a ETF will get you a relative share of the ETF holdings, but the actual holdings are not up to you then.