Sometimes there's no other way than to specify locants of the both carbons of the double bond, when the single locant specification would be ambiguous. Especially when there's predefined fixed numbering, like in natural compound skeletons:
pinane
pin-2(10)-ene
(*older name layout:* 2(10)-pinene)
(See usage in chemical papers: Cyclobutane ring opening of pin-2(10)-ene with mercury(II) salts. A new, high-yield synthesis of p-mentha-1,8-dien-7-ol or Some free-radical addition reactions of pin-2-ene, pin-2(10)-ene, and thuj-4(10)-ene.)
Another example:
Δ6a(10a)-tetrahydrocannabinol
(*sometimes with incorrect punctuation:* Δ6a,10a-tetrahydrocannabinol)
(more on this notation also in Q: Use of Δ (delta) to describe alkenes)
This type of locant is called compound locant in IUPAC Nomenclature of Organic Chemistry (Preferred names (2013)) terminology:
P-14.3 LOCANTS
P-14.3.1 Types of locants
Traditional types of locants are arabic numbers, for example, 1, 2, 3; primed locants, for example, 1′, 1′′′, 2′′; locants including a lower case Roman letter, for example, 3a, 3b; italicized Roman letters, for example, O, N, P; Greek letters, for example, ‘α-, β-, γ-’ and compound locants, for example, ‘1(10)’ and ‘5(17)’.
(…)
When using general IUPAC rules for locant number minimization, one might be tempted to use the compound locant for some rather simple cases like multi-substituted cyclohexenes, like the 2nd wrong(?) name in:
cyclohex-6-ene-1,2,4-triol
(*not* cyclohex-1(6)-ene-1,2,4-triol)
(it's not the IUPAC citation, but result of almost all IUPAC naming software)
Why ‘cyclohex-1(6)-ene-1,2,4-triol’ is wrong might be concluded from the following rule
P-31 MODIFICATION OF THE DEGREE OF HYDROGENATION OF PARENTHYDRIDES
(…)
P-31.1 THE ENDINGS ‘ENE’ OR ‘YNE
(…)
P-31.1.4 Bi- and polycyclic von Baeyer parent hydrides
(…)
P-31.1.4.2 If there is a choice of names and numbering, the following criteria are considered in order until a decision is reached:
(1) a minimum number of compound locants. A compound locant is used for a double bond if the locants of the atoms at each end of the bond do not differ by a value of one. When a compound locant is required, the higher locant is cited in parentheses. A benzene ring is shown and described as a cyclohexatriene corresponding to the Kekulé structure. Other aromatic rings are treated similarly, When required;
Note: The seniority of single locants over compound locants was established in 1989 for numbering steroids [see S3-2.5(2), ref. 16] and extended to von Baeyer systems in the 1999 publication on von Baeyer nomenclature (see VB-8.3.1, ref. 7).
Examples:
bicyclo\[4.2.0\]-oct-6-ene \[not bicyclo\[4.2.0\]-oct-1(8)-ene\]
(…)
which however applies for polycyclic rings.
The mentioned ref 16 (IUPAC Nomenclature of steroids) section:
3S-2 Fundamental carbocycles, unsaturation and alkyl substitution at C-17
(…)
3S-2.5 Unsaturation
(…)
- If there is a choice of locants, single ones are preferred to compound locants, e.g. estra-5,7,9-triene is preferred to estra-5(10),6,8-triene.
So, IUPAC organic nomenclature rules tend to avoid these compound locants (x)y where it's possible. But, as Ivan Neretin has stated in his comment, for monocyclic compounds, there even is no ambiguity at all.
I'd add: … if there was a rule stating, that the compound locants for ‘ene’ position can be applied only for branches.