So whilst using the default siren I noticed an issue, whilst one wailer works, the second wailer doesn't seem to work whenever I press E, it's always the primary one. Furthermore, I once heard the second wailer activate on the default siren and LSPDFR immediately crashed (but I don't know if this is a coincidence). I'm looking through the lists of good sirens, but I'm just wondering is there something obvious I've missed when it comes to the sirens or is this a bug. 

 

Note: I activate the wailer by holding the E key whilst the siren is on.

 

When I say Secondary siren I mean the one at 0:16

 

Signed the absolute.

If I understand right, you have Wail and Phaser but you can't use Yelp using the default siren controls. There are a few of solutions for this that I know of:
1, Tap E once quickly and then hold it down, that will play the yelp tone with default controls making it so you don't need to download anything
2, Download a siren controller such as Siren Mastery or Dynamic Lighting System (it will work as a simple controller without configs) out of these two I recommend using DLS as it is simpler if loaded properly
3, Rather than default vehicles or Non-ELS use ELS vehicles, of course if you want to keep your game vanilla to keep access to online that would not work