What is "split jurisdiction"?

Section 6, Rule 135 of the present Rules of Court provides that when by law, jurisdiction is conferred on a court or judicial officer, all auxiliary writs, processes and other means necessary to carry it into effect may be employed by such court or officer.
If this Court were to sustain petitioners' contention that jurisdiction over their certiorari petition lies with the CA, this Court would be confirming the exercise by two judicial bodies, the CA and the CTA, of jurisdiction over basically the same subject matter – precisely the split-jurisdiction situation which is anathema to the orderly administration of justice.35 The Court cannot accept that such was the legislative motive, especially considering that the law expressly confers on the CTA, the tribunal with the specialized competence over tax and tariff matters, the role of judicial review over local tax cases without mention of any other court that may exercise such power. Thus, the Court agrees with the ruling of the CA that since appellate jurisdiction over private respondents' complaint for tax refund is vested in the CTA, it follows that a petition for certiorari seeking nullification of an interlocutory order issued in the said case should, likewise, be filed with the same court. To rule otherwise would lead to an absurd situation where one court decides an appeal in the main case while another court rules on an incident in the very same case. (G.R. No. 175723; February 4, 2014)

Basically, split jurisdiction is a situation where the law seemingly provides appellate remedies to different courts. This is against public policy because it encourages forum shopping and conflicting rulings between courts.