Protip: Don't try to backstab the engi then sap the sentry you would have to be really good and really fast to do this.
Stab, straight away press q to switch to sapper and click madly.
And hope you made your last weapon your sapper.
i run to the dispenser saying medic and just sit there for a bit then wait for the right time to sapnstabi normally works well if no one walks into youthis is easier if the sentrys facing the other way