NP spam servant

Artoria for NP spam, Lancelot for crits.

Also you should consider teams other than double Tamamo, even for arts spam (Tamamo+Waver for example is often better).

Both options would work just fine. However, Lancelot (Saber) is an arts Servant that would prefer both Critical and Arts boosts rather than just Arts. In such a situation, you would preferably run a different servant than two Tamamo. One should be more than enough. Lancelot's NP does hurt, but he only has a decent Arts boost baked into his kit when he NPs when it comes to NP damage.

Regardless, when it comes to stacking buffs, you preferably want to mix it up a little. Card buffs and Attack buffs for example stack multiplicatively. A double Arts buff from Tamamo would grant +100% Arts boost and therefore the damage of an Arts attack would be doubled. However, if we instead grant one attack boost for 50% and one Arts boost for 50%, the damage would would amount to (1.5*1.5 = 2.25) = 125% damage boost. Besides, a different option would give you more flexibility in what bases you cover.

For both Altria Archer and Lancelot (saber) you preferably do want to keep them well fed with critical-related skills. Altria wants extra stars while a sufficiently levelled Lancelot can provide his own stars and would rather have more critical damage. Fragments is a great choice in any case for the supports.

In terms of CEs, Another Ending is very much superior to Formal Craft for Lancelot (Saber). For Altria, you can go either way depending on what you wish to focus on critting or NP spamming, but Another Ending is pretty nice on her too.

If you want ideas for some other servants who can dish out arts damage in arts based teams, you could also look into Li Shuwen and Orion. Li’s 3rd skill boosts his arts damage and he can hit through dodge and evade. Orion has a 1 turn attack boost built into her first skill and can deal significant damage against any servant, not just the males. As the others said, I’d consider running Tamamo with Waver instead of dual Tamamo. Or have another Arts booster, Helena, in your back support that you can swap in for that last boost to damage.

If you want more of a meme team, there is an interesting infinite Rider Mordred np combo that runs double tamamo, allowing for a Mordred NP every turn. I'm sure if you check youtube for it you will find it. It does require a mlb kscope and a mlb prima cosmos, but the result is really funny.