I get a lot of questions about practicing modern Witchcraft and Hellenismos at the same time. I, well, I'm not for that. In that link, you will find my generalized reasons why, based on the ancient ways, but I want to show you why it doesn't work (in my opinion, of course) from a modern standpoint.

Here is a disclaimer first: I am not an authority on anything but I have been a practicioner and teacher of Hellenismos for nearly five years now and before that, I practiced and taught as an Eclectic Religious Witch. I was initiated in a long running line here in The Netherlands and practiced in a coven. I taught 'meditation' and 'history of witchcraft' classes and was quite active both online and in my local community, organising rituals for the eight yearly feasts. The point is: I know both sides of the coin.

I came across a Tumblr post a while ago. It read the following and shows exactly where the rub is with combining the two, because this is actually very accurate in terms of modern Witchcraft:

"Tips for writing spells:

If you want to keep something close, bury it in your back yard.
 If you want to attract something, bury it under the front door step
 If you want to destroy its influence, burn it.
 If you want it to move away and sink, throw it in running water
 If you want to disperse it to a distance, throw it into a crossroads
 If you want to fix its influence, inter it in a five-spot pattern
 If you want it to work by means of spirits, bury it in a graveyard
 If you want to hide its point of origin, conceal it in a tree
 If you want it to work in secret, give it in food or drink
 If you want it to work by stealth, hide it in clothing or on objects
 If you want its influence to begin or strengthen, throw it East
 If you want its influence to end or weaken, throw it West
 If you want its influence to rise and fall cyclicly, float it in a tidal estuary"

Now let me make the Hellenic version for you, which won't get you spells but the religious and mythological 'rules' of our religion:

"If you want remove something from your life and household, bury it in your back yard.
 Don't ever bury anything under the front door step, it'll keep miasma trapped
 If you want to sacrifice something to the Theoi, burn it.
 Don't polute any body of water, this is where the Naiad house
Sacrifice to Hekate at a crossroads for fortune and purification for your family
 A five-spot pattern means nothing in Hellenismos
 Graveyards are miasmic. Spirits need to be appeased but never 'worked with' to prevent miasma
 Don't polute any tree or woodland, this is where the Dryads house
 Giving anything in food or drink might give you a belly ache if it's not to be consumed
 Hiding anything in clothing or on objects will do absolutely nothing
 East is the territory of Euros and considered unlucky
 West is the territory of Zephyros, the gentlest of the winds, the fructifying wind and the messenger of spring
 The ancient Hellenes were sailors and knew that if it ended up in the sea, it stayed in the sea. If it was alive before, it died"

So, literally every single point of truth in modern Witchcraft has the reverse truth in Hellenismos. This is why, in my opinion, it simply does not work to practice the two simultaniously. It can't be done in good concience. Even if you practice the two without overlap, it would two entirely different mindsets that you would have to switch between any time you practice one of the two. I believe--I practice--Hellenismos as a way of life. It's soaked into everything. Because of that, I had to get out of Witchcraft once I progressed into Hellenismos. I hope this clarifies my views some.