Can I still set up several testamentary trusts from one will to multiply the low tax brackets?

Not for the tax-splitting reason people used to use it. Before 2016, each testamentary trust created by a will was taxed on its own graduated brackets, so setting up several separate testamentary trusts from one estate could multiply access to the lowest rates, a popular strategy at the time. Since 2016, the Income Tax Act has taxed most testamentary trusts the same way it taxes trusts created during someone's lifetime: at the top marginal rate, with no basic personal exemption, no matter how many separate trusts a will creates.

You can still legally set up multiple trusts from one will for other, non-tax reasons, for example, to give different children control over different property, to separate a trust for a disabled beneficiary from one for other children, or to keep a business succeeding through its own trust structure, but you generally shouldn't expect that structure alone to multiply low-bracket access anymore. The narrow exceptions that still get graduated rates are a graduated rate estate for a limited period after death, and a qualified disability trust for an eligible beneficiary.

If your existing will was drafted around the old multiple-trust tax strategy, it's worth having it reviewed, since the tax rationale behind that structure no longer holds up.