I am not sure if I completely understand what you mean by making your dreads thicker. Are you wanting to combine your dreads? If your dreads are 12 years old......each individual mature dread can not thicken. Adding any compound such as honey is really not a good idea. And lemon juice is a big no no for dreads because it damages the hair strand from the inside out. You can combine 2 dreads together with hemp or beads to hold them together and they will eventually start growing together from the scalp. That is referred to as congos.
I often congo baby dreads (dreads formed from new growth) to existing dreads to thicken my dreads at the roots. I have lots of new growth that forms baby dreads. But joining a baby dread to a mature dread only thickens the roots...not the whole dread.