Can you you have let's say a 1 year working holiday visa in Netherlands and a 1 year working holiday visa in Germany, Belgium, Estonia etc. at the same time? or can you only have one at a time?
Unlike short-term Schengen visas, long-term visas in the EU are regulated by each member state individually. This means that you can have as many long-term visas for Europe as you want, without anyone batting an eye. There are some exceptions to this, but they only kick-in for permanent residencies and family members of the EU.