Is it hard to do 2 sided SMD PCB in oven?

No, it’s generally not a problem. You can solder the light parts first and then flip it and solder the rest. The surface tension of the solder will hold small parts onto the board.

In the unusual case where you might have a heavy part or two on the bottom on the second cycle you can add adhesive or solder it later by hand.

'Heavy' relates to the relationship between pad area and weight, with a rule of thumb limit of about 30g/in^2 of pad area. So, for example, a 1206 resistor with 0.005in^2 of pad area has a limit of about 0.15g. The actual mass is 10g/1000 or 0.01g each.

See this accepted answer for references.

It is done as you mentioned using adhesive tape on already soldered side.

Also, components should be selected carefully and heavy component should be put on 2nd side if possible.

[EDIT] : My mistake only some low cost operations do the tap on bottom side to support heavy parts. Typically reflow profiles is carefully curated and SMT JIG is used after one side is done to hold up components. This supports heavy parts.

_{Image source: My own photo}

The correct answer is to check with whomever is doing your SMT assembly and ask them.

Double-sided boards are very common. But your own vendor has to be setup to do it.