if you choose one level of demonslayer before reaching level 5 in sacred huntsmaster, at level five sh you will be precluded from choosing outsiders as a new favored enemy (which makes sense), but you will not be able to choose whether to increase outsiders or your new favored enemy choice by +2. you will have two favored enemy categories both providing +2 bonuses.
however, if for example at level six, after having chosen 5 levels of sacred huntsmaster and identified outsiders as your first favored enemy choice, you add a level of demonslayer, your to hit and damage bonuses against demons will be +4. subsequent increases in the sacred huntsmaster favored enemy feature will work as anticipated with the option to increase bonuses against outsiders.
choosing demonslayer before level 5 sacred huntsmaster locks you out of the option to increase favored enemy bonuses against demons via the sh favored enemy feature, but again, this only happens if you choose demonslayer levels before sh level 5.