We have three elections at the same time (national, regional, communal), and the solution is simple: make the slips for each election have different colors. The

We have three elections at the same time (national, regional, communal), and the solution is simple: make the slips for each election have different colors. The national election slips are white, communal slips are blue etc. To make sure nobody takes puts e.g. national slips in all three envelopes there's a small cutout at the edge of the envelope that shows the color of the slip inside without revealing the vote.