How to solder two-sided PCB
There's no problem running it through the reflow process twice, once with parts on each side.
Things like resistor networks, resistors, capacitors, most chips will stay on fine. See this answer for a rule-of-thumb on component mass vs. pad area (30gm/in\$^2\$)- as I stated my standard 0805 is 4gm/in\$^2\$ so plenty of margin. No need of any fancy solder alloys.
Get the type of stencil printer that allows you to print paste on a PCB with components already installed on the opposite side, or make a jig.

There are also adhesives designed to hold parts into place, typically used when one side of the board must be wave soldered. They can be applied by pneumatic dispenser, or by stencil or screen printed (photo from the manufacturer):

Generally total assembly costs will be lower if you can avoid placing parts on both sides of the PCB, but sometimes that is not possible.